“Passing it for the first time should start setting off serious alarm bells,” said Julie Brigham-Grette, a geoscientist at the University of Massachusetts Amherst.

And yet, there’s far from consensus among climate scientists for how loud to ring those alarms or just how much this measurement actually matters.

There is something undeniable about this moment, although Harvard climate scientist Daniel Schrag saidthat doesn’t mean the planet has passed “a point of no return where it’s too late.”

The reality: “1.5 C is bad, 2 C is worse, 2.5 C is much worse,” and so on, he said. “It always can get worse. And therefore, there’s no point where you throw up your hands and you say, ‘Oh, we failed, we’re done.’”

To be sure, one year of passing the 1.5 Cthreshold does not indicate that global temperatures are stuck that high. It’s a worrying sign for sure, but 2024 could be a blip.Climate is what happens over decades — not what happens in an individual year.

Many climate scientists have also long been clear-eyed that hitting 1.5 degrees of warming was inevitable. “I could tell 20 years ago that we were going to pass this threshold,” said Schrag, co-director of Harvard’s Science, Technology, and Public Policy Program.

What the goal signaled, more than anything, was an aspiration, a target that, though likely unachievable, provided a benchmark for global ambition. A 2016 report from the United Nations said it was set “not as a scientific question of feasibility, but rather as a moral imperative of necessity.”

That target was agreed toback in 2015 when world leaders gathered in Paris for the United Nations annualglobal climate talks. They consented to hold the Earth’s temperature to well below 2 degrees above preindustrial times and to aim for the 1.5-degree goal.

That more ambitious target was especially important to low-lying island nations — the ones the most at risk from increased sea level rise and least responsible for burning large amounts of fossil fuels that drive climate change.

Meanwhile climate change continues to outpace even the best models.

“Frankly, how quickly it’s warmed over the last couple years has been quite surprising,” said Peter Huybers, a professor in the department of Earth and Planetary Sciences at Harvard University. In 2023 and 2024, in particular, temperatures warmed faster than anticipated, likely due to decreased cloud cover over the oceans, which leads to more heat from the sun being absorbed by the Earth.

The cause for the decreased cloud cover is currently being debated, Huybers said — perhaps it is natural variability, perhaps it is the unintended consequence of a ban on a particularly dirty kind of shipping fuel, which caused emissions that could increase cloudiness.

Since Paris, the goal has shifted. A 2023 report found that overshooting the 1.5-degree target was “fast becoming inevitable” as nations failed to cut carbon emissions quickly enough.

Now,the goal is to minimize just how long the planet spends above that threshold, through rapidly eliminating greenhouse gas emissions and ramping up technologies to remove carbon from the atmosphere. Only taking that second step can push climate change in reverse, as heat-trapping gases, such as carbon dioxide, remain in the atmosphere for hundreds of years.

As of now, such carbon-removal technologies are expensive and only exist on a small scale, which means there is no clear mechanism for eliminating emissions already in the atmosphere. The best path forward, according to scientists, is to curb further carbon pollution as quickly as possible and let temperatures hold steady at an elevated level from preindustrial times.

There are some positive signs of that, said Max Holmes, CEO of the Woodwell Climate Research Center in Falmouth. Globally, emissions are expected to peak soon, as clean energy is rapidly being adopted and dirty power plants are taken offline. Still, progress is measured in decades.

“It’s going to be a long time until temperature plateaus and then heads the other direction — probably not in our lifetime — but emissions will and that is something to celebrate,” he said.

In the meantime, “the science of this is not going away,” said Adam Schlosser, deputy director and senior research scientist at the MIT Center for Sustainability Science and Strategy. Continuing to emit greenhouse gases only leads to one thing: rising temperatures. And while the yearly average may bob above and below 1.5 degrees in the coming years, it will eventually stay above the threshold, likely by the middle of the 2030s, Schlosser said.

Still,what’s at stake with the target of 1.5 degrees is whether the ambitious nature of the goal will ultimately be recognized as something that motivated nations around the world to step up the speed of their response to the climate crisis, or whether failure to achieve it will lead to the public perception of failure — and, worse, an excuse to give up, said Holmes.

“If people think it’s game over, then you lose all motivation to do what needs to be done.”

Sabrina Shankman can be reached at sabrina.shankman@globe.com. Follow her @shankman.